The radiating diaphragm of a loudspeaker typically vibrates axially, with one side thereby creating pressure waves outside the loudspeaker enclosure. At certain frequencies, there are natural structural resonances in the diaphragm and other moving parts. When the diaphragm is driven by the voice coil, these resonances may be excited. They correspond to peak displacements of the diaphragm, but other than the main resonance (at which the diaphragm may move pistonically) the displacements are in the form of dynamic bending deformations. These deformations affect the magnitude and directivity of the radiated pressure, and are highly frequency dependent. They therefore adversely affect the sound of the loudspeaker. The lowest frequency at which this occurs is known as the “breakup frequency” of the driver.
For desirable radiated pressure the driver breakup must be controlled in one or more of a number of possible ways. The material or geometry of the deforming part may be designed for high stiffness, to increase the breakup frequency to above the working range of the driver. Alternatively, the material of the deforming part may be selected for high damping in order to reduce the magnitude of the deformation at resonance.
These approaches to controlling breakup operate in different ways, and so the most suitable approach will depend on other factors such as size, shape, working frequency range, moving mass target, cost etc.
Geometries that are commonly used to increase stiffness include cones and domes; their curvature gives them much greater stiffness compared to a flat diaphragm. Stiffening ribs may be added to the geometry; these generally protrude perpendicular to the diaphragm rear surface and extend in the direction of a resonance deformation to increase the frequency of that resonance.
Factors such as build height, moving mass target, costs and cosmetics may mean that diaphragms with the geometries above cannot give high enough breakup frequencies. This is especially true where the diaphragm must be flat to fit the driver within a shallow enclosure.